Fix testtool device registration

Fix testtool device registration

The NETCONF testtool functionality for automatic bulk
registration of devices was broken for some time.

We have decided to not only fix the problem but also
refactor testtool the following way:

- Create request bodies using NormalizedNodes
instead of using config templates.

- Register devices with PATCH instead of POST
to allow merging nodes into topology and prevent
error for consequent batch requests.

JIRA: NETCONF-796
Change-Id: Ifd7c74e9b07bb56b84e6a1ffea0bf2809b85246c
Signed-off-by: Illia <illia.ihushev@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>